The present invention relates to an elevator of the type in which a cage is moved upward and downward by a hoisting device.
An elevator of this type is provided with a brake which holds a cage in its stop position when the cage is stopped, and safely brakes and stops the cage in the event of an emergency such as a power failure during the travel of the cage. This brake includes shoes which are pressed against a drum under a constant force by a mechanical means, such as a spring, and a frictional force produced at this time brakes or holds the cage. Generally, the frictional force of the brake is the product of the friction coefficient and the pressing force, and the friction coefficient is non-linear and is a function of the sliding speed and the pressing force. Therefore, in order to make the friction coefficient stable, it is required to select a suitable combination of the materials for the drum and the shoe, the optimum pressing pressure, etc. For running the elevator, this pressing force is electrically released, and the cage is driven by a motor or the like.
Such conventional devices are disclosed, for example, in International Publication WO86/03184 and Japanese Patent Unexamined Publication No. 60-148879.
When the elevator is designed to travel at high speed, the range of the sliding speed is widened, and the frictional force of the brake varies substantially according to the conventional method. Namely, with the conventional structure which merely applies the pressing force of a constant level mechanically, variations in the frictional force becomes large due to the wide range of the sliding speed, so that it is difficult to achieve a stable braking even if the combination of the materials of the element is suitably selected. Therefore, the braking acceleration is varied, so that the braking distance is increased, or in contrast a large braking shock is produced. Particularly in the type of structure in which the cage is driven by a sheave through a rope, if the braking force is too large, a slip develops between the rope and the sheave, so that the cage may fail to be braked effectively. The slip between the rope and the sheave also shortens the lifetime of the rope.
As the stroke of travel of the cage becomes long, the weight of the rope and other associated parts increases. Therefore, an unbalanced weight becomes smaller relatively but then the inertial mass to be braked becomes large. Therefore, even though the force required for holding the cage in its stop position is small, a large braking force is required. Namely, the braking force becomes much larger than the force for holding the cage in the stop position. Therefore, when it is intended to produce such a relatively large braking force by a mechanical means such as a spring, the size of the device becomes large.